Straight trucks and tractor-trailers are often used as a means to transport cargo that must be maintained at a predetermined or desired temperature during transportation in order to preserve the quality of the cargo. This cargo may be food, agricultural goods or various other temperature critical items. Hereinafter, any vehicle that transports cargo that must be maintained at a predetermined temperature during transportation will be referred to as a transportable vehicle or a transport vehicle. Hereinafter, the term cargo means any item that is transported, stored or otherwise supported within a conditioned space of a transport vehicle that must be maintained at a predetermined temperature.
Transport vehicles that transport such temperature sensitive cargo include a conditioned space. A temperature control unit controls the temperature within the conditioned space within a temperature range surrounding a selected thermal setpoint. Some transport vehicles have a plurality of conditioned spaces that may be maintained by the unit at the same thermal setpoint or varying thermal setpoints.
The temperature control unit can be programmed to cool or heat the conditioned space to the thermal setpoint. Frost can build-up on the temperature control unit when the temperature control unit is cooling the conditioned space. More particularly, frost, otherwise known as condensate, can build-up on an evaporator coil of the temperature control unit and decrease the efficiency of the temperature control unit while in the cooling mode. Defrost cycles are typically utilized to remove the condensate. A defrost cycle is performed by circulating a heated fluid through the evaporator coil. The heat from the fluid is passed to the evaporator coil, causing the condensate to melt. The temperature control unit returns to the cooling mode after defrosting and cools at a greater efficiency due to the decreased build-up of condensate on the evaporator coil.
Many varying types of defrost cycles exist, such as manual defrost cycles, fixed time defrost cycles and static pressure defrost cycles. A person manually initiates and terminates a manual defrost cycle, while the fixed time defrost cycle initiates and terminates on a fixed timer that is programmable with a time period. The static pressure defrost cycle initiates when the static pressure across the evaporator coil reaches undesirable levels and terminates when the static pressure is at a desirable level.